The biological treatment of waste water for the removal of BOD involves the use of one or more partially submerged bodies forcibly rotated to expose the organisms on the contact surfaces to the atmosphere for the absorption of oxygen. The growth of the biological slimes on the surfaces of the rotating contactor is thereby encouraged. Such rotors are of substantial size and oftentimes are ten to twelve feet in diameter and lie horizontally so that the axis of rotation is adjacent the level of the waste water in the tank. Approximately the lower half of the rotor is immersed in the waste water and the upper half is out of the waste water and exposed to the atmosphere or air.
Such rotary biological treatment rotors have been known in several forms in the past as illustrated in U.S. Pat. No. 3,894,953 which generally describes the biological treatment process, but does not describe in detail the nature of the contactors or rotors. British Pat. No. 935,162 discloses the use of banks of plates or discs of plastic, approximately a half inch thick and mounted on an axle to be turned through the waste water in a tank. U.S. Pat. No. 3,777,891 describes the revolving discs in a biological treatment apparatus as being constructed as foamed synthetic resin such as foamed polystyrene. Such discs have also been indicated to be of styrofoam, and held together by tie bolts and spacers to allow waste water to pass between the discs.
British Pat. No. 1,333,061 also suggests the use of non-porous or woven mesh or with perforations formed by punching, expanding, or molding.
Other disc configurations in such biological treatment systems have been variously shaped, as in U.S. Pat. No. 3,997,443 wherein the discs have peripheral ribs on their opposite faces; in U.S. Pat. No. 3,827,559 formed plastic sheets of polystyrene or polyethylene lie transversely of and on the central axle and cooperate with each other to define radially extending and concentric passages to induce trickling and movement of the waste water through the discs as they revolve. My copending application executed Oct. 21, 1976, discloses polyethylene or polystyrene plastic sheets up to 12 feet in diameter and tied together with tie rods on a central shaft wherein the discs have cup-like recesses over their entire faces.
In U.S. Pat. No. 3,904,525, plastic discs are formed with various configurations for being spaced apart, and the discs are mounted in banks spaced from the rotation axis around which the discs orbit and are carried entirely downwardly into the waste water tank and then entirely removed. This patent also illustrates such discs in segment shapes rather than in entirely circular shapes.
British Pat. No. 1,397,664 discloses another rotor arrangement wherein plastic filter packs, which are rectangular and box-like in shape with openings and/or channels extending therethrough, are mounted in spaced relation to a rotation axis so that the filter packs swing down through a body of waste water to be entirely immersed and entirely removed from the waste water. U.S. Pat. No. 3,847,811 discloses a rotor with plates spaced from each other and lying substantially normal to radii of the axis. The plates are mounted on a rotor frame to be immersed entirely into the waste water and then entirely removed.
All of the prior art and other previously known arrangements have distinct problems because auxiliary structural support means are necessary in order to hold them in desired positions. Such auxiliary support structure, whether it be tie rods, or brackets or frames, will deteriorate over a period of time and will necessitate a considerable amount of maintenance which, in a large piece of equipment such as these rotors, can involve a considerable amount of effort, time and expense, and of course, the system must be shut down while such maintenance is being carried out. Furthermore, the previous rotors have not made efficient use of the available space so as to obtain a maximum exposure of the waste water to air when the wetted media emerges from the waste water.